1. Technical Field
The present invention relates to a sensor for sensing a parameter such as strain or temperature; and, more particularly, to a fiber Bragg grating sensor system for detecting a resonance dip in a fiber Bragg pi-shifted grating optical signal.
2. Description of Related Art
FIG. 1 illustrates a fiber Bragg grating system known in the art and generally indicated as 10 for the measurement of a wavelength of a fiber Bragg grating. The fiber Bragg grating system 10 includes a broadband source 12, a coupler 14, a fiber Bragg grating 16 and a wavelength detection system 18. The wavelength detection system 18 may be a scanning filter (e.g. Fabry-Perot) based concept, or a scanning interferometer (Michelson, Fourier Transform) based approach. The system output is a measure of a spectral reflectance of the fiber Bragg grating 16, which is typically a single resonance peak as shown in FIGS. 1B, 1C.
It is known in the art to use Bragg grating based transducers for the measurement of pressure using a number of xe2x80x9cpressure-to-strainxe2x80x9d transduction techniques. Of the approaches developed, a glass-collapsed based compression sensor configuration provides a stable, low hysteresis form of transduction. With this high stability sensor configuration, a limiting factor in determining the accuracy of the overall sensor system appears to be in the ability of the wavelength detection instrumentation system to resolve and track small wavelength shifts of the sensor.
Current techniques used, such as scanning Fabry-Perot and Wavemeter (FFT Spectrometers), have resolution capabilities of about 0.05 to 0.1 um in 1550 nanometer range. For a sensor with a scale factor typical of that developed for grating based devices, this wavelength shift resolution can correspond to a pressure resolution between 0.05 to 0.2 pounds per square inch (psi). In certain applications, particularly pressure transient build-up monitoring, the pressure may need to be monitored with an even higher degree of resolution. In this case, the accuracy of the measurement may not be of utmost importance, but resolution and repeatability are required.
In its broadest sense, the present invention provides a fiber Bragg grating sensor system having a pi-shifted fiber Bragg grating sensor in combination with a dual resolution mode wavelength detection system.
The pi-shifted fiber Bragg grating sensor responds to a parameter, and further responds to an optical signal, for providing a pi-shifted fiber Bragg grating sensor signal containing information about the parameter.
The dual resolution mode wavelength detection system responds to the pi-shifted fiber Bragg grating sensor signal, for providing a dual resolution mode wavelength detection signal containing information about the parameter.
The dual resolution mode wavelength detection system includes a low resolution wavelength detection system for detecting the overall centroid resonance peak in the pi-shifted fiber Bragg grating sensor signal; and includes a high resolution wavelength detection system for detecting a resonance dip in the resonance peak of the pi-shifted fiber Bragg grating sensor signal.
The high resolution wavelength detection system includes a scanning laser for detecting the resonance dip in the resonance peak.
The dual resolution mode wavelength detection system may also include a high resolution wavelength detection system for detecting a high resolution spectral feature in a resonance peak of the pi-shifted fiber Bragg grating sensor signal.
The scanning laser repeatably scans a pi-shifted fiber Bragg grating profile, and either identifies a resonance dip center wavelength during each scan, or seeks and locks onto the resonance dip center wavelength.
The dual resolution mode wavelength detection system is switchable for detecting either the resonance peak or the resonance dip.
In one embodiment, the fiber Bragg grating sensor system has a broadband source for providing the optical signal in the form of a broadband optical source signal.
Alternatively, the fiber Bragg grating sensor system has a tunable laser for providing the optical signal in the form of a tunable laser optical signal. In this embodiment, the dual resolution mode wavelength detection system also has a reference detection system that responds to the tunable laser optical source signal.
In effect, a tailored (pi-shifted) grating resonance is used to enhance the detection capability of the fiber Bragg grating sensor. Moreover, the present invention provides a means for facilitating the measurement of a tailored grating sensor element using a dual optical interrogation schemexe2x80x94one being the xe2x80x9cconventionalxe2x80x9d one used now, and the other a high resolution mode. These interrogation systems interrogate the same sensor, but with differing degrees of accuracy and resolution.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.